Or maybe only 127 or 32 or 18 per individual controller.
The USB standard supports up to 127 devices, and USB hubs are a part of the standard.
Many modern USB 3.0 hosts use Intel XHCI USB controllers, which impose their own limit on the total number of endpoints to 96. This means that the maximum number of devices able to be attached to an Intel XHCI host controller - or an XHCI-based host system - is actually 96 endpoints / 3 endpoints per device = 32 devices.
If we use 8-port USB 3.0 hubs to connect devices to our host machine, this means that you could attach 18 total devices:
HUB #1 (1 hubs * 6 devices / hub) + 8 attached devices = 14 devices +
HUB #2 (1 hubs * 6 devices / hub) + 8 attached devices = 14 devices +
HUB #3 (1 hubs * 6 devices / hub) + 2 attached devices = 8 devices
= 32 total devices for XHCI
But what has this to do with Roon? Roon might not limit the number of watched folders, but the burden of watching that many might make the core unstable.
On the other side, if one needs a NAS, one should use a NAS and not trying to abuse a Nucleus/ROCK installation for this purpose. Having just one share that provides storage of many storage devices together might quickly become cumbersome and impractical to use.